The woman President Trump retweeted videos from this morning, Jayda Fransen, is the deputy leader of a far-right, ultra-nationalist group known as Britain First. In its mission statement, the group calls itself "patriotic resistance" to what it sees as increasing "Islamization" of the U.K. and Europe.

Since its founding in 2011 by former members of the British National
Party, Britain First has organized demonstrations against what it calls "Islamic extremism" and "mass migration" to the U.K. Members of the group have held “Christian patrols” in parts of London where Muslims live and visited
refugees and migrants at camps in Calais, France to urge them not to
come to the U.K. The group says it opposes the "colonization of our
homeland" through immigration.